Thermistor Types – Their Workings and Applications

A thermistor is a temperature sensing element composed of sintered semiconductor material that exhibits a large change in resistance in proportion to a small change in temperature. A thermistor can operate over a wide temperature range and give temperature value by its resistance change, which is formed by two words: Thermal and resistor. The Positive temperature coefficients (PTC) and the negative temperature coefficient (NTC) are the two major thermistor types that are used for temperature-sensing applications.

Thermistors are easy to use, inexpensive, sturdy and respond predictably to a change in temperature. Thermistors are mostly used in digital thermometers and household appliances, such as ovens and refrigerators, and so on. Stability, sensitivity and time constant are the general properties of thermistor that make these thermistors durable, portable, cost-effective, highly sensitive and best for measuring single-point temperature.

Thermistors are of two types:

1. Positive Temperature Coefficient (PTC) Thermistor
2. Negative Temperature Coefficient (NTC) Thermistor

PTC Thermistor

PTC thermistors are resistors with a positive temperature coefficient, in which the resistance increases in proportion with the temperature. These thermistors are differentiated into two groups based on their structure and the manufacturing process. The first group of thermistor comprises silistors that make use of silicon as a semi-conductor material. These thermistors can be used as PTC temperature sensors due to their linear characteristics.

Switching type thermistor is the second group of PTC thermistor that is used in heaters, and also the polymer thermistors come under this group which are made up of plastic and are often used as resettable fuses.

Types of PTC Thermistor

PTC thermistors are classified based on the temperature level they measure. These types depend on the following:

• Elements: These are of disk, plate and cylinder types of thermistors.
• Lead, Dip type: These thermistors are of two kinds, viz. painted and non-painted. These have high temperature coatings for mechanical protection, environmental stability and electrical insulation.
• Case type: These can be plastic or ceramic cases that are used based on the application requirement.
• Assembly type: This is unit product due to its construction and shapes.

Typical Characteristics of PTC Thermistor

The following characteristics of thermistors show the relationship between the various parameters like temperature, resistance, current, voltage and time.

1. Temperature Vs Resistance

In the below figure, we can observe how fast the resistance varies with temperature, i.e., an abrupt rise in resistance with little changes in temperature. PTC exhibits a slight negative temperature coefficient over the normal temperature rise, but at higher temperatures and Curie point, there is a sharp resistance change.

2. Current\Voltage Characteristics

This characteristic shows relationship between voltage and current in a thermal equilibrium state, as shown in the figure. When the voltage increases from zero, the current and temperature also rise until the thermistor reaches a switch point. Further increasing the voltage leads to decrease in the current over an area of constant power.

3. Current Vs Time Characteristics

This tells the reliability required for solid state switches in heating and protection against the high-current applications. When more than given voltage is applied to a PTC thermistor, large amount of current flows in the instant of voltage application due to low resistance.

Applications of  PTC Thermistor

1. Time delay: Time delay in a circuit provides the time needed for a PTC thermistor for sufficient heating to switch from a low-resistance state to a high-resistance state. Time delay is dependent upon the size, temperature and the voltage to which it is connected as well as to the circuit employed in. These applications include delayed switching relays, timers, electric fans, etc.

2. Motor starting: Some electrical motors have a startup winding that needs to be powered only when the motor starts up. When the circuit is turned on, the PTC thermistor has less amount of resistance, allowing current to pass through the startup winding. As the motor starts, the Positive Temperature Coefficient thermistor heats up, and – at one point, switches to a high-resistance state, and then it terminates that winding from the mains power. The time needed for this to occur is based on the required motor start up.

3. Self regulating heaters: If there is a current passing through a switching Positive Temperature Coefficient thermistor, then it will stabilize at a certain temperature. It means that if the temperature decreases, in proportion to the resistance, allowing more current to flow, then the device gets heated. If the temperature increases to a level that limits the current passing through the device, the device gets cooled.

PTC thermistors are used as timers in the degaussing coil circuit of CRT displays. A degaussing circuit using PTC thermistor is simple reliable and inexpensive.

NTC Thermistor

A thermistor with a negative temperature coefficient means that the resistance decreases with a rise in temperature. These thermistors are made from a cast chip of semiconductor material such as a sintered metal oxide.

Most commonly used oxides for these thermistors are manganese, nickel, cobalt, iron, copper and titanium. These thermistors are classified into two groups depending upon the method by which the electrodes are attached to the ceramic body. They are:

1. Bead type thermistors
2. Metalized surface contacts

Bead type thermistors are made of platinum alloy, and lead wires that are directly sintered into the ceramic body. Bead-type thermistors offer high stability, reliability; fast response times and operates at high temperatures. These thermistors are available in small sizes and exhibits comparatively low dissipation constants. These thermistors are normally achieved by connecting them in series or parallel circuits. Bead type thermistors include the following types:

• Bare Beads
• Glass Coated Beads
• Ruggedized Beads
• Miniature Glass Beads
• Glass Probes
• Glass Rods
• Bead in Glass Enclosures

The second group of thermistors has metalized surface contacts that are made available with the radial or axial leads as well as without the leads for mounting – by means of spring contacts. A variety of coatings are available for these thermistors. The metalized surface contact can be applied by painting, spraying or dipping as required and the contact is fixed into a ceramic body. These thermistors include the following types:

• Disks
• Chips
• Surface mounts
• Flakes
• Rods
• Washers

Typical Characteristics of NTC Thermistor

There are three electrical characteristics that are taken into account for all the applications in which NTC thermistors are used.

• Resistance-Temperature characteristic
• Current-Time characteristic
• Voltage-Current characteristic

1. Resistance-Temperature Characteristics

NTC thermistor exhibits the negative temperature characteristics when the resistance increases with the slight decrease in temperature, as shown in the figure.

2. Current-Time Characteristics

The rate change of current is low due to the high resistance of the thermistor. Finally, as the device approaches an equilibrium condition, the rate of the current change will decrease as it reaches the final value of time which is shown below, in the figure.

3. Voltage-Current Characteristic 

Once a self-heated thermistor reaches to an equilibrium condition, the rate of heat loss from the device is equal to the power supplied. In the below figure, we can observe these two parameters’ relationship wherein, we can observe a decrease in voltage at 0.01 MA current and again the voltage increases at a peak current of 1.0 MA, and then decrease at the current value of 100 MA.

Applications of NTC Thermistor

1. Surge Protection: When a NTC thermistor is turned on, it absorbs the surge current across the equipment and protects it by changing its resistance.

2. Temperature Control and Alarm: NTC thermistor can be used as a temperature control system or temperature alarm system. When the temperature increases, and the resistance of the thermistor decreases – the current becomes high and gives alarm or switches the heating system on.

These are the two major thermistor types used for different temperature sensing applications. Hope that the thermistor characteristics and applications, in addition to the types, might have given you a better & wholesome understanding of the topic or electrical and electronic projects. Please write your suggestions and comments in the comment section given below.

Photo Credits:

Thermistor types by ussensor
PTC Thermistor by paumanokgroup
Current\Time characteristics by hiel
Voltage\Current characteristic: by cantherm